364
CHEMICAL ENGINEERING
It should be borne in mind that a multiple effect, where evaporators are connected in series, has the same capacity as a single effect, provided the initial steam, pressure and vacuum are the same. However, the steam consumption is reduced in proportion to the number of effects installed and this increased economy is the only reason for installation of multiple effects, which are naturally much more expensive than single-effect evaporators. In some cases, evaporators are arranged parallel with individual steam connections and condensers, and it is evident that the capacity will increase with the number of evaporators, but the steam consumption will also rise in the same proportion,, In the first case, the total temperature fall T has been divided into a number of smaller temperature falls, TI, Tz, TZ) etc., and the evaporating capacity of the multiple effect will be Q = K0 (T^ + T2 + T3, etc.) = K0T, which is also the capacity of a single effect. In the second case, the capacity of each evaporator, Qn ~ KoTn and therefore the total output of all the evaporators equals the sum of all Q's, Q = K0T
Steam Consumption.—It is apparent that any evaporating system should have complete heat balance and that in all cases the amount of heat entering and leaving the evaporators should be the same. If we call
S — B.t.u. in steam admitted to steam chest, V — B.t.u. in vapor going to condenser,
Ri, Rz, Rs = B.t.u. lost by radiation in first, second and third effect, W = B.t.u. in weak liquor entering evaporator, H = B.t.u. in heavy liquor leaving evaporator, and Ci, C%, <?3 = B.t.u. in condensate discharged from evaporators. The heat balance of a single effect would be
and for a triple effect
S = V +
+
+ C78 + #,. + R, + #3 + H - W These equations give the factors that determine the steam consumption of an evaporator and -make it clear that all types of evaporators must have the same efficiency with reference to steam con-
\sumption provided the working conditions are identical. Therefore the claim of some manufacturers that their particular type of evaporator consumes less steam than other constructions is misleading as there can only be a difference in the losses by radiation, and with a good insulating covering these losses are so small that the difference is negligible.
In a single effect, the determination of V is simple, as it can readily be figured from the difference between the weight of the weak and heavy liquor, the temperatures and the vacuum. In a multiple effect, the problem is rather difficult as for the determination of V it is necessary to know the quantities of water evaporated in each effect. The evaporation in each effect is not the same, but will be smallest in the first, and largest in the last effect, on account of the fact that the liquor passing from one evaporator to the next will give off some of its heat (reevaporate). This vapor added to the vapor coming from the first effect will naturally increase the amount of evaporation in the second effect, and so on. The exact amount of reevaporation depends on
H Ci
FIG. 3.—Heat balance in single effect.